Page 63 - Physics 10_Float

GEOMETRICAL OPTICS

ii. The ratio of the sine of the angle of incidence ‘i’ to the sine of the angle of refraction
sin i
‘r’ is always equal to a constant i.e., = constant.
sin r
sin i
where the ratio is equal to the refractive index of the second medium with
sin r

respect to the first medium.

i.e., sin i = n
sin r

This is also called Snell's law.
.
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 The angle of incidence for which the angle of refraction becomes 90 is called critical
angle. When the angle of incidence becomes larger than the critical angle, no
refraction occurs. The entire light is reflected back into the denser medium. This is
known as total internal reflection of light.
 A simple microscope, also known as a magnifying glass, is a convex lens which is
used to produce magnified images of small objects.
 A compound microscope is used to investigate structure of small objects and has
two converging lens, the objective and the eyepiece.
 Telescope is an optical instrument which is used to observe distant objects using
lenses or mirrors. A telescope that uses two converging lenses is called refracting
telescope. A telescope in which the objective lens is replaced by a concave mirror is
called reflecting power telescope.
 The magnifying power is defined as “the ratio of the angle subtended by the image as
seen through the optical device to that subtended by the object at the unaided eye”.
 The resolving power of an instrument is its ability to distinguish between two closely
placed objects.
 The ability of the eye to change the focal length of its lens so as to form a clear image
of an object on its retina is called its power of accommodation.
 The disability of the eye to form distinct images of distant objects on its retina is
known as nearsightedness. The nearsighted eye can be corrected with glass or
contact lenses that use diverging lenses Light rays from the distant objects will .
diverge by this lens before entering the eye.
 The disability of the eye to form distinct images of nearby objects on its retina is
known as farsightedness. This defects can be corrected with the aid of a suitable
converging lens. The lens refracts the light rays more towards the principal axis
before they enter the eye.

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